[Codeine seeking behavior in rats using a weight-pulling method].

The purpose of this work was to examine the development of preference for codeine and codeine-seeking behavior by using an apparatus in which rats were compelled to pull against a weight to gain access to codeine-admixed food but not in the case of normal food. Rats developed a strong preference for codeine-admixed food by the repetition of choice and forced trails. The preference for codeine-admixed food was maintained at levels between 60 to 80% for 14 days without any forced trial. After rats developed this preference, they were tested as to whether they might pull against the weight in the apparatus. We observed that the rats did pull at most 40-120 g/100 g body weight in order to reach the drug-admixed food. This is defined as maximum pulling weight. As the weight load was increased, the rats showed more frequent weight pulling behavior for the codeine-admixed food while the duration of eating decreased. The maximum weight pulled was more than 70 g/100 g body weight in 4 out of 6 rats. These results suggest that the degree of drug-seeking behavior in rats can be detected by the use of the weight-pulling method.